INSTRUMENTATION Strong's new approach to ruling of gratings, possibilities of semiconductive particles suspended in an insulating fluid, and detector systems are discussed
bu Ralp H. Mϋller iiBEnx G. Ingalls writes a fascinating t*- and well illustrated article on ruling engines in the current Scientific Ameri can [186, 45 (1952)]. Although it is written for the intelligent layman, it contains most of the outstanding prob lems and describes the present efforts being made to improve grating construc tion and performance. I t should be required reading for all those who use gratings and especially for those who would light-heartedly offer to improve gratings. Ingalls discusses in entertaining and informative manner the "seven demons that bedevil the ruling-engine worker." They a^e friction, wear, warpage, creep, vibration, dust, and temperature changes. These devils "never stay exorcised and the battle against them is perpetual." The author has ex pressed error sources and their magni tudes with enviable clarity and direct ness. I t is one thing to speak of microinch tolerances, but when we recall that the swaying of a tree outside the build ing introduced periodic errors, or that a 15-hour "dry run" of the machine is re quired to bring the machine's lubricat ing films to uniform thickness, one's respect for the problem increases rap idly. .
more than a century ago, Kowland laid the foundation for dependable gratings in 1882. The account of all the im provements which have been made since that time is impressive reading. In it, one can witness all his own pet ideas sys tematically demolished—they have all been tried! Perhaps the only revolu tionary idea, which is being studied intensively at MIT, is the suggestion of George R. Harrison, which is to give up the intention of further improving the ruling engine and to build an error-cor recting servomechanism into the ma chine. The principle here, which we dis cussed in a previous column, is to use an interferometer as the displacementmeasuring element and to actuate the cutting tool after a definite number of fringes have passed, including minute fractions of a fringe. The idea is old, but in the skillful hands of Harrison and his associates, it has shown great prom ise. Ingalls has also devoted a portion of the amateur scientist section of this issue (page 90) to a very thorough discussion of the labors of Wilbur Perry and David Broadhead, who have accomplished such marvels in the lapping of precision-feed screws. More of this is promised for the Julj' issue.
Ruling
Polaresistivity
nf
(irutings
The new approach to the ruling of gratings is that of John Strong, in which two precision screws shift the diamond point rather than the grating carriage. When ruling a line, the diamond point is stationary and the grating moves be neath the point. Although crude gratings were ruled
and
Polaristors
Chemists can have a field day in fol lowing the prospects and possibilities outlined in a paper on "Polaresistivity and Polaristors" by the illustrious H. E. Hollmann [Proc. Inst. Radio Engrs., 40, 538 (1952)]. These systems refer to semiconductive particles suspended in an insulating fluid. Fibration can be 19 A
induced by applied electric fields and the conductivity becomes nonlinear as soon as the fibration at a certain thresh old field reaches a point at which the semiconductive fibers bridge from one · electrode to the other. Such systems have an analog in the fluid magnetic clutch in which finely divided iron particles immersed in oil attain almost infinite viscosity when a magnetic field is applied and, therefore, very effectively couple two rotating steel faces. The difference, in the former case, is that the particles are polarized electrically instead of magnet ically. The simplest example is af forded by colloidally dispersed graphite in oil such as Oildag. When two elec trodes are immersed, the initial con ductivity is substantially zero, but as po tential is applied, fibration occurs and a conductivity appears which is a func tion of applied voltage. Some of this persists unless the fibers are destroyed by vibration or turbulence. Solid systems of this sort have been prepared with various powders dispersed in thermo- or cold-setting plastics. These have been termed "Polaristors." In one case, the use of titanium oxide of reduced oxygen content dispersed in Castolite revealed a 16-fold nonlinearity, which is far higher than that of Tliyrite. The author has given a hint of some of the more exciting possibilities in say ing: Moreover, it must be kept in mind that the polaresistive colloids are not only nonlinear, but are, at the same time, sensitive to temperature and me-
ANALYTICAL
20 A
IMPORTANT ANALYSES
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CHEMISTRY
INSTRUMENTATION chanical strain or stress, so t h a t new sensing elements and electromechanical transducers m a y be developed. Since we are completely free to intermix a great variety of powders with various properties, such as semiconductive ferromagnetic particles, photoelectric or photoconductive powders, and t h e like, the versatility of polaristors may be greatly enhanced. I t is curious, b u t undoubtedly significant, t h a t so much of promise in modern physics is related to the behavior of intentionally impure substances or heterogeneous systems. T h e analyst is always required, and ready, to specify these impurities. Perhaps, t h e physical chemist can be induced to broaden his horizon and look into these things. For all we know, the future g r a d u a t e s t u d e n t may no longer have to measure some thermodynamic property of a pure substance to four significant figures in order to become a doctor of philosophy, b u t m a y be encouraged to study "imp u r e " substances. T h e calculated risk— t h a t he m a y discover something new— is unlikely to frighten him. Related work on t h e production and properties of isotropic artificial dielectrics is described b y R. W. Corkum [Proc. Inst. Radio Engrs., 4 0 , 574 (1952)]. I n these systems, small and very uniform conducting spheres are embedded in an insulator such as Styrafoam in precise geometric spacing. T h e structural dielectric so produced has most interesting properties. T h e behavior of these materials is most conveniently studied in the microwave region. Detector Systems M o s t of us have come to take for granted the highly dependable performance of modern recording potentiometers. Among the several well known types, the Brown Electronik, the Leeds and N o r t h r u p Speedomax, and the Bristol D y n a m a s t e r , all make use of a chopper device or synchronous switch which converts a small d.c. signal into an alternating signal which is more conveniently amplified and applied to the servo restoring system. M u c h has been written about detector systems, b u t there is continued study of the problem in t h e incessant search for better signal-to-noise ratio and better all-around performance. A careful discussion of t h e requirements a n d design for a direct current null detector has been given by Frederick L. M a l t b y of
the Bristol Co. I t appears as a preprint available from the American Institute of Electrical Engineers, 33 West 39th St., New York 18, Ν . Υ . One question which M a l t b y has dealt with in great detail is t h e elimination or minimization of dissymetry effects in t h e inverter. For a discrimination ratio of 2000 to 1, t h e dissymmetry must be 0 . 5 % or less in order to prevent overloading due to q u a d r a t u r e voltage. This is difficult to do with semiresonant reed inverters for t h e fundamental rea son t h a t symmetry, in this class of in verter, depends upon making contact a t a consistent position. T h u s , t h e sym m e t r y is subject to variation with wear on any mechanical bias. One way in which the dissymmetry may be held to such a low value is by timing t h e action to t h e zero transitions of t h e supply line voltage. If the line voltage has only odd harmonics, t h e zero transition points are evenly spaced. E v e n harmonics are usually present to t h e extent of less t h a n 0.2% in most commercial power supplies. If t h e contact position is reversed a t these zero transition points or a t points in consistent relation to them, the contact action will be symmetrical. This principle has been applied b y moving an armature between two lam inated pole pieces which are polarized by a permanent magnet. T h e a r m a ture is energized by alternating current of peak value 10 times t h a t necessary to produce magnetic saturation of t h e ar m a t u r e . T h e action on the a r m a t u r e is essentially a square wave force which reverses abruptly a t t h e zero current point. T h e resultant acceleration of t h e a r m a t u r e tip is a b o u t 200 " g " a n d t h e range of free travel of t h e tip is re duced to 0.0005 inch, which serves to fix the reversal point still more defi nitely. I t is encouraging to note the elegance and u n a b a t e d attention which are di rected to these studies. T h e require ments always exceed t h a t which is cur rently possible and it is likely t h a t we shall witness m a n y new approaches to t h e ideal modulator. T h e recording of the diverse phenomena required by the analyst presents unusual demands on a modulator, with various input imped ances, frequency requirements, e t c . So far, instrument manufacturers have done their very best to keep pace with these unusual demands.
